1. Technical Field of the Invention
The present invention is generally related to the design and use of transponders (i.e., tags). The present invention is also related to methods and systems for remotely determining the location of tags. The present invention is also related to methods and systems for remotely communicating with tags. More particularly, the present invention is related to radar-assisted location of and communication with tags to obtain data.
2. Background
It is well known that the combination of transponder and radar-signal-processing technique can allow for the geolocation of the tag and uplinking of data from the tag. U.S. Pat. No. 5,486,830, issued Jun. 23, 1996 to R. Axline, describes such methods and systems. U.S. Pat. No. 5,486,830 describes a basic concept of initially locating the tag in a xe2x80x9cgeographical locationxe2x80x9d mode and then uplinking data from the tag in an xe2x80x9cuplink communicationsxe2x80x9d mode. The patent also describes how suppression of radar clutter can be obtained. Finally, the patent describes details of the structure of the phase-coded signals that the tag returns to the radar, as well as the method the radar uses to process the returned signals (echoes).
While U.S. Pat. No. 5,486,830 provides a useful description of phase-code signaling methods, radar-signal-processing techniques, determining tag geolocation, and tag data-uplinking, a problem in the art surrounding clutter suppression still persists. U.S. Pat. No. 5,486,830 discusses communication between a tag and radar at a common frequency with unique methods directed to the suppression of stationary clutter energy, accomplished primarily via tag modulation and radar processing. The present inventor has developed several xe2x80x9cin-bandxe2x80x9d frequency modes since U.S. Pat. No. 5,486,830 issued. In this context, the term xe2x80x9cin-bandxe2x80x9d denotes the meaning that retransmitted signals coming from the tag reside in the same band of frequencies as the band occupied by the original radar pulse. These developments mitigate the effects of clutter on a system""s ability to image the tag or uplink data from it. Yet, adequate clutter reduction still prevails as the dominant problem in radar-tag communications.
Based on the foregoing, the present inventor has found that quality limitations in radar-tag communication can be overcome using what may be now be referred to as xe2x80x9cout-of-bandxe2x80x9d frequency communications techniques, which will be further described herein.
In order to address the shortcomings of the prior art, the present invention provides improved systems and methods for geolocating and decoding data from tags.
Accordingly, it is a feature of the present invention to enable radar to transmit a very accurate reference frequency that a receiving tag can utilize as described in the following paragraph.
The reference frequency is embedded as information into a pulse signal transmitted by radar. The tag can extract the reference frequency from the radar pulses and use it to develop a second xe2x80x9coffsetxe2x80x9d frequency, based on the reference frequency. The offset frequency is known to within a small Doppler offset frequency by receiving radar (which can also be the transmitting radar).
In accordance with another aspect of the present invention, a tag is provided having the means to receive radar pulses containing reference frequency information; determine a second, offset frequency; translate the center frequency of the radar pulse by an amount equal to the determined offset; modulate, pulse by pulse, the electrical phase of each translated pulse; and transmit a geolocation sequence and/or tag data for receipt by a radar.
Data obtained from tags (tag data) can include, for example, information regarding the environment surrounding the tag, tag-related internal or operational data and/or messages associated with the tag""s location.
In employing the xe2x80x9cout-of-bandxe2x80x9d communication technique, the second, offset frequency, should be chosen to be sufficiently large that the frequency spectrum of the pulse signals retransmitted by the tag do not overlap the frequency spectrum of the radar pulse originally transmitted by the radar.
One primary advantage of the out-of-band function is that stationary clutter coming from passive scatterers in the radar scene is nearly completely removed from the processed echo, so that the transponding tag echoes need only compete with thermal noise, and not the clutter.
A second and novel benefit of the out-of-band function, as described herein, is that the radar will be able to geolocate the tag with an accuracy and precision commensurate with that obtainable by a state of the art synthetic aperture radar (SAR); e.g., precision of a fraction of a meter and accuracy on the order of a few meters. The out-of-band tag system of the invention achieves this accuracy and precision while also effectively suppressing all stationary radar clutter.
The above and other aspects of the invention are achieved as will now be further described. The present invention discloses methods and systems, including program products thereof, which enable both accurate and precise geolocation of tags and uplinking of data from tags. A system for out-of-band communication between radars and tags includes a tag receiver for receiving radar pulses, at least one processor for processing data captured from a sequence of radar pulses and at least one transmitter for transmitting tag data back to a radar. A tag can include in-band and/or out-of-band modes. In an xe2x80x9cout-of-bandxe2x80x9d mode, the tag can transmit tag data to the radar utilizing a frequency band that does not overlap with radar pulse frequencies. One method for transmitting tag data to a radar includes steps of receiving a radar pulse signal at a tag, very accurately translating the radar pulse signal to an out-of-band center frequency, and transmitting tag data back to the radar at the out-of-band frequency. A radar, given the teaching of the present invention, is capable of tuning its receiver to a frequency different from its pulse transmit frequency, thus allowing the radar to receive and process the out-of-band tag responses.